chaging mcastport is not supported when running in a cman managed cluster.
The correct way to add interfaces and change the multicast address is to use the cman directives, not corosync ones.
To change the multicast address use
<cman> <multicast addr="address"/> </cman>
To add interfaces (and this is unsupported) use the procedure document here:
http://sources.redhat.com/cluster/wiki/MultiHome